In general, a vehicle lamp can switch between a low beam and a high beam. The low beam is for illuminating a near area with a predetermined illumination intensity, and there are regulations on light distribution so as not to give glare to oncoming vehicles or preceding vehicles. The low beam is used mainly when the vehicle runs in an urban area. Meanwhile, the high beam is for illuminating a front wide range and a distant area with a comparatively high illumination intensity. The high beam is used mainly when the vehicle runs on a road where there are few oncoming vehicles and preceding vehicles. That is, although the high beam is superior to the low beam in visibility for drivers, the high beam has a problem that the high beam gives glare to walkers or drivers of vehicles existing in front thereof.
Recently, an adaptive driving beam (ADB) technology has been proposed for dynamically and adaptively controlling the light distribution pattern of a high beam based on the surrounding situation of the vehicle. The ADB technology is for detecting whether there is a preceding vehicle, an oncoming vehicle or a walker in front thereof, and suppressing glare to be given to those vehicles and walkers, for example, by reducing light to be radiated toward areas corresponding to the vehicles and walkers.
A vehicle lamp having an ADB function will be described. FIG. 1 is a block diagram illustrating a vehicle lamp having an ADB function according to a comparative art. It is noted that that this comparative art should not be considered as a prior art.
A vehicle lamp 1r includes a light source 10 and a driving device 20r. In an ADB, a high beam irradiation area is divided into N sub-areas (wherein N is a natural number). The light source 10 includes a plurality of light emitting units 12_1 to 12_N which are associated with the N sub-areas, respectively. Each light emitting unit 12 includes a semiconductor device such as a light emitting diode (LED) or a laser diode (LD), and is disposed so as to irradiate a corresponding sub-area. Each light emitting unit 12 may be a single device, or may include a plurality of devices connected in series.
The driving device 20r controls the plurality of light emitting units 12_1 to 12_N such that the respective light emitting units are turned on or off, thereby changing the light distribution of a high beam. Alternatively, the driving device 20r performs pulse width modulation (PWM) control on the light emitting units 12 at a high frequency, thereby adjusting effective luminance.
The driving device 20r includes a current source 30, a plurality of bypass circuits 40_1 to 40_N, and a controller 50. The current source 30 receives a battery voltage VBAT (also referred to as input voltage VIN) from a battery 2 through a switch 4, and stabilizes a drive current IDRV to flow in the light source 10, at a target amount.
The plurality of bypass circuits 40_1 to 40_N are associated with the plurality of light emitting units 12_1 to 12_N, respectively. Each bypass circuit 40 is configured to be switchable between an ON state and an OFF state. If an i-th bypass circuit 40_i becomes the ON state, the drive current IDRV flows in the bypass circuit 40_i, not in the light emitting unit 12_i, thereby turning off the light emitting unit 12_i. Meanwhile, if the bypass circuit 40_i becomes the OFF state, the drive current IDRV flows in the light emitting unit 12_i, thereby turning on the light emitting unit 12_i. 
An upstream processor 6 (for example, an electronic control unit (ECU)) for controlling the vehicle lamp 1r determines sub-areas to be irradiated by the high beam based on the situation of the front of the vehicle. Then, the processor 6 issues a control command to the controller 50 of the driving device 20r. The controller 50 controls the states of the bypass circuits 40_1 to 40_N based on the control command from the processor 6. Specifically, the controller 50 selects light emitting units 12 corresponding to the sub-areas to be irradiated, and turns off bypass circuits 40 parallel to the selected light emitting units 12 while turning on bypass circuits 40 parallel to the other light emitting units 12.
If a bypass circuit 40 is suddenly switched from the ON state to the OFF state, an output voltage VOUT of the current source 30 decreases. In a case where the current source 30 is configured by the topology of a back converter, a boost converter, a flyback converter, a forward converter or the like having a high-capacity smoothing capacitor to be connected in parallel to a load, if the output voltage VOUT suddenly decreases, electric charge accumulated in the smoothing capacitor is released, whereby the drive current IDRV flowing on the light emitting unit 12 side overshoots. On the contrary, if a bypass circuit 40 is suddenly switched from the OFF state to the ON state, the drive current IDRV undershoots. If the width of fluctuation of the drive current IDRV followed by turning on or off of a bypass circuit 40 is large, the reliability of the light emitting unit 12 is adversely affected, or a noise component increases. Particularly, in a case where a plurality of bypass circuits 40 are turned on or off at the same time, the width of fluctuation of the output voltage VOUT increases, and this problem becomes more remarkable. In order to solve this problem, there has been proposed a technology for gradually switching the bypass circuits 40 between the ON state and the OFF state (see JP-A-2008-126958).